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CARL D. LA RUE AND H. H. BARTLETT 
to an understanding of mass mutation. Certainly there is likely to be 
some causal relationship between such closely associated phenomena. 
One must of course take into consideration the possibility that the 
defective seeds represent zygotes of f. typica that failed to develop. 
Reckoned from the total number of seed-like structures sown, rather 
than from the number of plants obtained from them, the proportion 
of mutations in the polymorphic progenies would not be at all unusual. 
We are not inclined to believe, however, that any such explanation is 
the right one. Why should the typica zygotes in one case develop 
into uniformly strong and viable embryos, but in another case, en- 
vironmental conditions remaining the same, fail to produce even 
mature embryos? Moreover, if there were no essential difference 
between uniform and polymorphic progenies other than the failure 
of typica zygotes to develop, why should the mutations found in the 
polymorphic progenies be characteristic of the latter? It may be 
urged that the evidence is not sufficiently clear that the non-mass- 
mutant individuals might not throw mutations semialta, debilis and 
hilonga if grown in sufficiently large cultures. For the present it 
must suffice to say that they have not done so, although we are keenly 
aware of the fact that the cultures have not been as large as one would 
wish for convincing evidence on this point. Very much larger cultures 
to test this question are planned for next year. It should be remarked 
that the mutations of Oe. Reynoldsii are not sufficiently characteristic 
in youth to admit of accurate classification, and that consequently 
every plant of each culture must be carried to maturity if it is to be 
certainly identified. With most of the other mutable species it is 
possible to discard many of the typical individuals which make up the 
bulk of the cultures without giving them garden space, since the 
young plants are as easily distinguished as the mature ones. 
In marked contrast with Oe. Reynoldsii, all individuals of f. typica 
in the mass-mutant strain of Oe. pratincola seem capable of throwing 
the mutations characteristic of mass mutation in that species, and such 
individuals differ among themselves as widely as possible in degree of 
mutability. Moreover, in Oe. pratincola the number of abortive seeds 
seems to vary in approximately inverse proportion to the number of 
typica individuals obtained from the seeds. This fact might be adduced 
as an argument for considering the bad seeds as resulting from the 
abortion of typica zygotes. We do not wish to minimize this possibility 
but prefer for the present the hypothesis that the zygotes which fail 
to develop represent mutational types of excessively weak constitution. 
